如何在 flutter 中使用 sqflite 保存嵌套列表？

Question

Here I want to add bookmark functionality in my flutter news application. I get data from APIs . I display that data below.

This Image shows you how I get data from APIs

I am using this snippet for saving data with SQflite which I display below. I save this file with name bookmark_db_provider.dart .

import 'dart:io';

import 'home_screen_data.dart';
import 'package:path/path.dart';

import 'package:path_provider/path_provider.dart';
import 'package:sqflite/sqflite.dart';

class DBProvider {
  static Database _database;
  static final DBProvider db = DBProvider._();

  DBProvider._();

  Future<Database> get database async {
    if (_database != null) _database = await initDB();

    return _database;
  }

  initDB() async {
    Directory documentsDirectory = await getApplicationDocumentsDirectory();
    final path = join(documentsDirectory.path, 'ProductData.db');

    return await openDatabase(path, version: 1, onOpen: (db) {},
        onCreate: (Database db, int version) async {
      await db.execute('CREATE TABLE ProductData('
          'id INTEGER PRIMARY KEY,' //id
          'categoryName Text,' //headline
          'publisherName Text,'  //description
          'isAvailable Text,'      //content
          'categoryImgUrl Text'  //image
          ')');
    });
  }

  createProductData(ProductData productData) async {
    await deleteAllProductData();
    final db = await database;
    final res = await db.insert('ProductData', productData.toJson());

    return res;
  }

  Future<int> deleteAllProductData() async {
    final db = await database;
    final res = await db.delete('DELETE FROM ProductData');

    return res;
  }

  Future<List<ProductData>> getProductDataList() async {
    final db = await database;
    final res = await db.rawQuery("SELECT * FROM ProductData");

    List<ProductData> list = res.isNotEmpty ? res.map((x) => ProductData.fromJson(x)).toList() : [];

    return list;
  }
}

So, I want to how to save data and get data this with SQflite database in flutter . How I accomplish this?

Answer 1

I might be unclear with your question, but according to what I understood,

You need to call the method of this provider with data that you want:

DBProvider.init() // Use this only one which when the application is instaled

After that, you can call these methods from anywhere to put and get data

DBProvider.createProductData(productData) //insert data

Get data

DBProvider.getProductDataList() //get data 

Answer 2

This is an old question and hopes you have gotten an answer to it. However, a lot of flutter dev using sqflite struggles with handling data in a nested array format of the type mentioned eg

``` {
  "id": 1,
  "name": "xyz",
  "images": [
    {
      "imageId": 1,
      "image": "image1"
    },
    {
      "imageId": 2,
      "image": "image2"
    }
  ]
} 
```

Since json handling is not part of sqflite at the moment, it is suggested to either; a., save inner array as a string/text field in the 'image' column of table 'data' like **

"[{"imageId": 1, "image": 'image1'}, {"imageId": 2, "image": 'image2'},}"

**, no guaranty.

or, b., flatten out the inner array so as to have only **

data[id, name, image1, image2, image3,...].

** this approach may be possible in a simple array as given but in a complex system. flattening out may be really cumbersome.

my suggestion, create two tables, one for data, and another for images. let each row of images table have reference or relationship with corresponding data table row. Your data class and images class will be something like,

```
class Data {
    int dataId;
    String name;
    List<Image> images;
    data({this.id, this.images, this.name});
    ...
``` 

and

``` 
class Image {    
    int imageId;
    int dataId;
    String image;
    Image({this.imageId, this.dataId, this.image});
    ...
    } 
```

Your sqflite table data will have only two fields 'dataId' and 'name' and the image table must include 'dataId' as the relationship between the two tables.

to save data, you can use transactions like

```
void saveData(Data data, Map<String, Object> map) async {
    await db.execute(""" INSERT INTO "data" (name) values (?) """, [data.name]);

// retrieve dataId of the new row inserted. last_inserted_rowid can also be used if the database does not contain several tables that may have been updated or saved before completing the transaction.
 
 int dataId;
    List<Map> x = await db.query('data', columns: ['dataId'], where: 'name = ?', whereArgs: [data.name]);
    dataId = x[x.length - 1]['dataId'];
   

    db.transaction((txn) async {
      var batch = txn.batch();
      data.images
          .map((e) => {
                batch.rawInsert(
                    """ INSERT INTO "images" (dataId,image,) values (?,?,?,? ) """, [dataId, e.image])
              }).toList();
    });
  }
 ```

to retrieve data and images, you can try something like

 ```
 Data _data = new Data();
 
 Future<void> fetchData() async {
    if (db != null) {
      // do not execute if db is not instantiate
      //retrieve all data from data table and store in instantiated, also instantiate images array as empty list
      final dataList = await db.query(data);
      _data = (dataList as List)
          .map(
            (data) => Data(
              dataId: data['dataId'],
              name: data['name'],
             images: data['images'] != null ? data['images'] : []
            )) .toList();
    
      _data.forEach((data)  async {if (data.images != null) {
        List<Image> _images = [];
        var imageResults = await db.query(images,where: 'dataId =?', whereArgs: [data.dataId]);
      
        _images = (imageResults as List).map((e) => Image(
          imageId: e['imageId'],
          dataId: e['dataId'],
          image: e['image']
        )).toList();
        _data.images.addAll(_images);       
      }  });

    }
  }
  ```
 

with that approach, you should be able to handle nested array in flutter and sqflite